Electrolytic apparatus for reduction of aluminum bromide



Jan. 23, 1951 w. G. LOVELL ETAL 2,539,092

ELECTROLYTIC APPARATUS FOR REDUCTION OF ALUMINUM BROMIDE Original FiledNov. 12, 1940 Jnoenkors Patented Jan. 23, 195?.

ELECTROLYTIC APPARATUS FOR REDUC- TIUN F ALUMINUM BROMIDE Wheeler G.Lovell and Nelson E. Phillips, Detroit, Mich, assignors to GeneralMotors Corporation, Detroit, Mich, a corporation of Delaware Originalapplication November 12, 1940, Serial No. 365,188, now Patent No.2,373,320, dated April 10, 1945. Divided and this application December23, 1944, Serial No. 569,486

6 Claims.

This application is a division of our copending application Serial No.365,188, filed November 12, 1940, now Patent No. 2,373,320, dated April10, 1945.

This invention relates generally to the electrolytic reduction ofcompounds of aluminum to produce metallic aluminum. More specificallythe invention has to do with the electrolytic reduction of a fusedelectrolyte containing aluminum bromide.

Among the objects of the invention are the following: to provide animproved method and apparatus for producing metallic aluminum, toprovide a convenient and practical electrolytic method and apparatus forthe production of aluminum that has more advantageous energy relationsthan present methods, to provide a process and apparatus for producingaluminum that reduces the cost as compared with present practice, toprovide a practical and economical method and apparatus for producingaluminum of a high degree of purity; to provide im rovements in a methodand apparatus of producing substantially pure aluminum which makes itpossible to utilize economically a wider variety of cheaper aluminumcontaining raw materials than present commercial processes; to provideimprovements in a method and apparatus for recovering substantially purealuminum as it is formed by the electrolytic reduction of the aluminumcompound in the electrolyte; to provide improvements in anodes andcathodes, especially those for use in the electrolytic reduction ofaluminum bromide. Other objects and advantages of the invention willbecome more apparent as the description proceeds. Reference is herewithmade to the accompanying drawing forming a portion of this specificationin which the figure illustrates somewhat diagrammatically one form ofapparatus adapted to carry out the method in accordance with theinvention.

General description The device illustrated comprises a means for heatingand continuously circulating through a generally closed system a fusedelectrolyte of suitable boiling point containing aluminum bromide, airand water being excluded from the system. The fused, non-aqueouselectrolyte passes through an anode which directs the electrolyte inproper relation with respect to a vibratory cathode. The device is soconstructed and controlled as to deposit the aluminum at the cathodein afinely divided form and to remove it therefrom and cause it'to flow withthe elec- 2 trolyte past the cathode to a chamber where it is removed.In order to insure that the aluminum is deposited in a form that can bereadily removed by vibrating the cathode or readily carried by theflowing electrolyte, special addition agents may be provided for theelectrolyte. In order to make the electrolyte sufiiciently conducting,ingredients such as sodium bromide or potassium bromide form a portionor" the electrolyte. In the device illustrated the aluminum particlesare filtered from the electrolyte by means of a filter type piston, butother methods for separating the solid and liquid may be used.

Detailed description In the drawing is a reservoir in adapted to containthe electrolyte. A suitable heating means [2 provides sufiicient heat tomake up for heat lost from the apparatus and to maintain the electrolytein the chamber at the required temperature necessary to keep theelectrolyte'in a liquid state when the apparatus is not being operated.Make-up electrolyte may be added to the reservoir from time to time bymeans of opening l4 and cover It. Electrolyte is pumped from thereservoir by any suitable pump, which in the form shown is constructedas follows. Extending into the electrolyte in the reservoir is a glasscylinder I8, the lower end of which is in flow communication with theelectrolyte by means of openings 20. In the cylinder adjacent theopenings is an impeller 22 formed of tantalum. The impeller is securedto a glass rod 24 which is adapted to be rapidly rotated by the electricmotor 26. Rotation of the impeller during operation thus causes a flowof the electrolyte continuously through cylinder I 8 to an outlet 28,thence to the electrolytic cell proper, indicated generally by thereference numeral 3E The electrolyte enters the cell and passes throughan electrically conducting cup-shaped anode 32 having a series ofopenings 36 in the lower end thereof. Preferably the anode is formed ofcarbon. Other insoluble conducting anodes may be used. The openings areso arranged as to direct the electrolyte downwardly towarda cathode 36.v

In the form shown the cathode is a rod of tantalum curved to form aring, the ring being joined to a rod 38 mounted for oscillation within asleeve 4i) adjustably secured to one wall 42 of the cell. The sleeve androd pass through the wall. The rod 38 is adapted to be rapidly vibratedor ocked back and forth within the sleeve by ri'leans' of rod 44 fixedthereto at one end, connecting rod 65 pivoted at the opposite end of rod46, eccentric t3 electric motor 50. The rapid rocking or vibration ofthe cathode removes the aluminum therefrom in finely divided form. Thealuminum tends to'deposit in the form of feathery trees or dendrites andadvantage is taken of this. Certain addition agents may be added to theelectrolyte to control more fully the deposition and cause the formationof the aluminum in finely divided form. The source of electric currentis connected in any desired manner to the anode and cathode. Thepositive side of the current source may be connected to a flange 31 ofthe anode, for example, while the other side of the current source maybe connected to arm 44 connected to the cathode.

The aluminum particles flow with the circulating electrolyte from theelectrolytic cell through the outlet 56 and through the tube 58 to afilter chamber indicated genera ly by the reference numeral 63. Withinthe filter chamber is a piston element 62 of carbon or other suitable.material, said element having openings 54 in the head thereof. Above thehead of the piston is a filter element 55. In the device illustrated thefilter element is a glass fabric. The purpose of the glass filter andthe openings in the head of the piston is to separate the aluminumparticles from the electrolyte and to cause the particles of aluminum tocollect on the filter element. By means of a glass rod 58 connected tothe head of the piston, the piston and filtering element with theparticles of aluminum thereon are raised above the inlet in the filterchamber to bring the aluminum particles in contact with a second pistonHi within the filter chamber. In the form shown the piston ll! is ofcarbon. The aluminum particles are pressed together and form a cake ormass H which adheres to the piston 10, and thus removes the cake fromthe active filter area so as to permit better flow and also to keep thecake away from the electrolyte which may contain small amounts ofbromine and might thus otherwise dissolve some of the metal. Mechanicalmeans may be used for suspending the cake from piston H1. The cake ofmetal is gradually built up by successive additions and preferablyextends above the level of the electrolyte, thus permitting theelectrolyte to drain off. The piston it may be removed from time to timeby means of a tubular glass rod l2 secured th reto, a cover 14 of thefilter chamber being removed during the removal of the piston 70, andall or a portion of the"cake of aluminum may be removed from the piston'50.

The strained electrolyte leaves the filter chamber through an outlet 15and returns to the heated reservoir Iii by means of tube Hi. In the formof apparatus illustrated, the pump 22, etc., moves the electrolyte tothe cup-shaped anode and the flow of electrolyte from this point to thereservoir 18 is due to the force of gravity acting thereon.

As the aluminum is set free at the vibrating cathode, bromine isliberated at the anode, which is centrall arranged in the upperportioncf the cell. The free bromine boils up through the hotelectrolyte. It is important to maintain high temperatures so that thebromine will not be too soluble in the electrolyte and temperaturesapproaching the boiling point of the bromine-free electrolyte aredesirable. Under these conditions the aluminum halide from theelectrolyte wi l boil ofi with the bromine, and to make a partialseparation of thegtwo andreturn. the aluminum bromide-to .the cell, thevapors. pass-through a reflux column. This comprises a glass tube orreceptacle 8i] nearly filled with glass rings or beads 82. The top ofreceptacle is closed by a glass cover 'El. Surrounding a portion ofreceptacle so is an air chamber 84 having an air outlet 35 and an airinlet tube 33. Most of the vaporized aluminum bromide leaving the cellis condensed by the condenser-reflux column and flows back into thecell. A small amount of the aluminum bromide and the bromine pass out ofthe reflux-condenser through an outlet 9% arranged near the top thereof,and by means of a passage 92 enter a glass receiver 9 A water condenser96 condenses any uncondensed bromine and aluminum bromide vapors andreturns the same to the receiver.

The design of the anode is important. The bromine must be evolvedrapidly in order to avoid the formation of a gaseous film whichincreases the electrical resistance of the cell. The anode must bedesigned to be as close to the cathode as possible in order to keep thecell resistance at a minimum. Some circulation of electrolyte past theanode is necessary in order to avoid the formation of a layer of lowconductivity; too much stirring is to be avoided as it promotes thesolution of the bromine in the electrolyte with the consequentredissolving of the aluminum. The electrical resistance of the anodeshould be as low as possible. Several forms of anodes have been used inaccordance with the above requirements, the form shown in the drawingbeing preferred. The bottom of the anode is turned to aradius so that itwill fit within the cathode and it has a streamlined surface for thefree removal of the'bromine.

It is important in the operation of the cell that the surface of theanode not be allowed to become a. poor conductor of electricity. Undercertain conditions, particularly after moisture has had access to theelectrolyte, a high resistance film may be built up on the anode. Forthis reason, the electrolyte must at all times be protected from theintroduction of even small amounts of water, in order to assurecontinuous satisfactory operation of the cell for long periods of time.

The requirements for the cathode are that it should be a good conductorand so shaped that the flow of electric current will be equallydistributed and so that it will not obstruct the flow of liquid or ofsolid metal particles. High current densities are applied so that thedeposited metal will be in a form which can be readily dislodged. Theupper limit of electrical current in any particular application isgoverned by the amount of heat generated and this is dependent upon theelectrical resistance of the cell. The amount of heat generated shouldnot be such as to cause too vigorous boiling of the electrolyte. Anotherfactor to be considered in the determi nation of the maximum current isthe energy efficiency desired. The electrical energy lost in theinternal resistance of the cell as heat, varies with the square of thecurrent, while the amount of metal produced varies directly as thecurrent. The current used in practice in the cell depends consequentlyupon the economic balance between pounds of metal produced per hour, andkilowatt hours per pound required to produce it.

In one form and size of apparatus constructed in accordance with theinvention from 5!) to 225 amperes of current have been passed throughthe electrolyte in a cell having a diameterof-six inches. Currentdensitiesas high as approximately 15,000 amperes per square foot ofcathode area have been used. Good current efficiencies in the range of80 to 90% have been consistently obtained.

- The composition of the electrolyte may vary considerably. It is atpresent preferred as a matter of experience and compromise that thesodium bromide (when this is used as the current carrier) be about 24%of the electrolyte, the balance being aluminum bromide. Lowerproportions of sodium bromide increase the resistance of theelectrolyte, while higher proportions reduce the resistance. Consideringthe electrical resistance alone, the ideal would be the largest possibleamount of sodium bromide. The temperature of the electrolyte preferablyshould be maintained as near the boiling point of the electrolyte as canbe in order to remove the bromine as rapidly as possible from thevicinity of the electrode and to keep the electrolyte as free of bromineas possible. In view of this, the lower the concentration of sodiumbromide the lower the permissible operating temperature, while thehigher the concentration of sodium bromide the higher the requiredoperating temperature. With the at present preferred bath com osition(24% sodium bromide and 76% aluminum brom de) an electrolyte temperatureof about 850 F. has been used with success. With an electrolyte composedof 19% sodium bromide and 90% aluminum bromide an operating tem eratureof about 509 F. may be used, while with an electrolyte compo ed of 20%sodium bromide and 80% alum num bromide an operating temperature of 650F. mav be used.

In order to ensure that the electrolyte flow through the cell is as nearstreamline flow as possible, the anode is so designed as to distributethe electrolyte substantially uniformly over the 'crosssection of thecell. In the form il strated the aluminum released from the cathodeflows downwards with the el ctrolyte. The rate of flow of theelectrolyte should be fast enough so as to continuously remove thealuminum from the cell and still not agitate the bath enough to causethe bromine to redissolve any appreciable amount of the aluminum.

It has been found desirable to add small amounts of certain substancesto the bath or electrolyte in order to control the size of the particlesof aluminum. Under some conditions of operatic-n, especially with verypure materias, .or on prolonged electro ysis of less pure materials, thealuminum forms mo s-like a gregations, which, when detached from thecathode, tend to agglomerate in large pieces of the mossy, p rous typeup to an inch or more in diameter. Such large pieces are undesirable inthat they do not flow through the cell properly since they are subjectedto diverse liquid currents and to mechanical striking by the vibratorycathode. As a resut they may be subject d to the action of bromine inthe cell and partially dissolved, thus resulting in a decrease in thecurrent efficiency of the cell. The large pieces also have a tendency toclog the connecting tubes to the filter chamber. We have found thatsmall amounts of such materials as naphthalene, phenanthrene, oranthracene when added to the electrolyte prevents the formation of thelarge particles of aluminum and thus obviates the above mentioneddisadvantages of such large particles. One part of the addition agent inabout a quarter of a million parts of electrolyte has proven sufiicient.The addition agent must be replenished from time to time as needed.Other addition agents which have been used are lubricating oil, rubberand the like. Although we do not wish to be bound by any definitetheory, we believe that the mechanism of the action is about as follows.When the addition agent is added to the hot electrolyte, itdecomposes-and forms insoluble colloidal carbon which is absorbed on theclean and active surfaces of the aluminum particles so as to keep theparticles from sticking or welding together to form large particles. Inaccordance with our theory organic materials which decompose in aluminumbromide at bath temperautres to form colloidal carbon may be usedefiectively; We prefer to use hydrocarbons or compounds containinglittle or no oxygen so as to prevent the formation of aluminum oxide asa final product to contaminate the bath.

Due to the fact that aluminum bromide solutions of bromine are verycorrosive to most met: als, especially at the high temperatures usedherein, it is necessary to use materials that are not attacked thereby.In the form of apparatus shown, the cell proper, the fi ter chamber, thereservoir and the passages connecting the same are formed of Pyrexglass. It is contemplated that other vitreous and ceramic materials, aswe as enamel surfaces may be used, as well as carbon when its electricalproperties permit. While it is at present preferred that the anode be ofcarbon it may be formed of tantalum. The best results have been obtainedwith a cathode formed of tantalum. It is contemplated that the cathodemay also be formed of carbon.

The cake of aluminum after removal from the filter chamber is acoherent, spongy mass which may be readily broken up. small amounts ofaluminum bromide and sodium bromide (when sodium bromide is used as thecurrent carrier). The cake of aluminum may be heated in an electricmuffle or other furnace to melt the aluminum. During the heating thesmall amount of aluminum bromide is distilled off and may be recoveredfor return to-the apparatus. The sodium bromide and the aluminum meltand the latter separates into a lower layer which may be drained off andmay be cast into pigs. Remarkably pure aluminum has been produced inaccordance with the invention.

We Wish it to be understood that we do not desire to be limited to theexact details of construction and operation shown and described, forobvious modifications will occur to a person skilled in the art.

We claim:

1. In a paratus for producing aluminum in solid, particulate form, aring-shaped cathode, means for rapidly vibrating said ring-shapedcathode, a cup-shaped anode located generally above said ring-shapedcathode and having its bottom portion curved to a radius to fit withinsaid ring-shaped cathode, said curved bottom portion of the anode havinga plurality of openings therein, and pumping means for continuouslypumping a fused, non-aqueous electrolyte into the upper portion of thecup-shaped anode and through said openings past the ring-shaped cathode.

2. In an apparatus for producing aluminum in solid, particulate formfrom a fused electrolyte containing aluminum bromide; an electrolyticcell having therein a ring-shaped cathode and a cup-shaped anode locatedgenerally above said ring-shaped cathode and having its bottom portioncurved to a radius to fit within the ring- It contains shaped cathode,said curved bottom portion having a plurality of openings therein; saidelectrolytic cell having an opening below said ringshaped cathode; meansfor rapidly vibrating said ring-shaped cathode; means for continuouslyflowing said fused non-aqueous electrolyte into the upper portion ofsaid cup-shaped anode and through the openings in the curved bottom ofsaid anode past the vibrating ring-shaped cathode and out through saidopening in the cell; and

means for passing electric current from said anode to said vibratingcathode to deposit aluminum at said cathode in solid, particulate form,the vibration of said cathode dislodging the aluminum particles from thecathode and the dislodged aluminum particles being carried out of thecell by the electrolyte flowing therefrom.

3. In an apparatus for producing aluminum in solid, particulate formfrom a fused, nonaqueous electrolyte containing aluminum bromide; aring-shaped cathode of tantalum, means for rapidly vibrating saidring-shaped cathode, an insoluble cup-shaped anode located generallyabove said cathode and having its bottom portion curved on a radius toflt within the ringshaped cathode, said curved bottom portion of theanode having a plurality of openings therein, means for continuouslyflowing a fused, nonaqueous electrolyte containing aluminum bromide intothe top of said cup-shaped anode and through the plurality of openingsin the curved bottom portion of the anode to a point beyond thevibrating ring-shaped cathode, means for passing electric currentthrough the electrolyte from said cup-shaped anode to the vibrating,ring-shaped cathode to liberate bromine at the curved lower surface ofthe anode and deposit aluminum at the vibrating, ring-shaped cathode insolid, particulate form, said vibration dislodging aluminum particlesfrom said cathode so that they are carried by the flowing electrolyte toa point beyond the ring-shaped cathode, and means for removing from theelectrolyte the liberated bromine.

4. In an apparatus for producing aluminum in solid, particulate formfrom a fused, non-aqueous electrolyte containing aluminum bromide; anelectrolytic cell having an opening in the bottom thereof, a ring-shapedcathode in said cell above shaped cathode, said curved bottom portion ofthe anode having a plurality of openings therein, means for continuouslyflowing a fused, nonaqueous electrolyte containing aluminum bromide intothe top of said cup-shaped anode and through the plurality of openingsin the curved bottom portion of the anode past the vibrating ring-shapedcathode and out said opening in the bottom of the cell, means forpassing electric current through the electrolyte from said cup-shapedanode to the vibrating ring-shaped cathode to liberate bromine at thecurved lower surface of the anode and deposit alumintu'n at thevibrating, ring-shaped cathode in solid, particulate form, saidvibration dislodging solid, aluminum particles from said cathode so thatthey are carried by the flowing electrolyte out of the cell through saidopening in the bottom thereof, and means for removing from the cell theliberated bromine. said cell being closed except for the opening in thebottom thereof for flow of electrolyte therefrom, the means for entr ofelectrolyte to the top of the anode and the bromine removing means.

5. In apparatus for producing aluminum in solid, particulate form from afused, non-aqueous electrolyte, a ring-shaped cathode, means for rapidlyvibrating said cathode, a generally cupshaped anode located generallyabove said cathode and having its bottom portion curved to a radius tofit within the ring-shaped cathode, said curved bottom portion having aplurality of openings therein, means for retaining electrolyte in thespace between the electrodes and means for introducing electrolyte intothe cup-shaped anode.

6. An apparatus as in claim 5 in which the ring-shaped cathode is oftantalum.

WHEELER G. LOVELL. NELSON E. PHILLIPS.

REFERENCES CITED The fo lowing references are of record in the f le ofthis patent:

UNITED STATES PATENTS Number Name Date 189,658 Schoepflin Apr. 11, 1877650,646 Long May 29, 1900 860,657 Hatfield July 23, 1907 995,476 McNittJune 20, 1911 1,186,306 Greenawalt June 6, 1916 1,251,302 I'ainton Dec.25, 1917 1,801,011 Koeppen Apr. 14, 1931 1,820,844 .Steinbuch Aug. 25,1931 1,897,308 Hunter Feb. 14, 1933 1,965,399 Wehe July 3, 19342,071,087 Philipp Feb. 16, 1937 2,151,599 J-aeger Mar. 21, 19392,218,021 Corneil Oct. 15, 1940 2,257,746 Janes Oct. 7, 1941 2,373,320Lovell Apr. 10, 1945 2,376,535 Fisher May 22, 1945 FOREIGN PATENTSNumber Country Date 16,475 Great Britain July 15, 1912 479,081 GreatBritain Jan. 31, 1938 557,386 Great Britain Nov. 18, 1943 556,017 FranceSept. 14, 1922

1. IN APPARATUS FOR PRODUCING ALUMINUM IN SOLID, PARTICULATE FORM, ARING-SHAPED CATHODE, MEANS FOR RAPIDLY VIBRATING SAID RING-SHAPEDCATHODE, A CUP-SHAPED ANODE LOCATED GENERALLY ABOVE SAID RING-SHAPEDCATHODE AND HAVING ITS BOTTOM PORTION CURVED TO A RADIUS TO FIT WITHINSAID RING-SHAPED CATHODE, SAID CURVED BOTTOM PORTION OF THE ANODE HAVINGA PLURALITY OF OPENINGS THEREIN, AND PUMPING MEANS FOR CONTINUOUSLYPUMPING A FUSED, NON-AQUEOUS ELECTROLYTE INTO THE UPPER PORTION OF THECUP-SHAPED ANODE AND THROUGH SAID OPENINGS PAST THE RING-SHAPED CATHODE.